Composite materials are increasingly being used for fabricating structural parts used in aircraft and other applications where the high strength-to-weight ratio and anisotropic nature of composites afford advantages over non-composites. For example, composite materials are being used or studied for use in aircraft wings, horizontal stabilizers, vertical stabilizers, and other parts of aircraft. Such composite parts can be made by first forming a preform of a plurality of non-impregnated plies of fiber material that are laid atop one another so as to form a preform structure having roughly the overall shape of the part to be produced. The preform is impregnated with a matrix material and cured in a mold to produce the finished part.
Some complex part shapes must be produced by assembling a preform from a plurality of component pieces. For example, in fabricating a preform for a composite wing having an outer skin reinforced by stringers and intercostals, the skin, stringers, and intercostals are formed separately. The preform is assembled by securing the stringers and intercostals to the skin and to each other, and then the preform is impregnated and cured to form the finished part. Various techniques can be used for securing the stringers and intercostals to the skin and each other. However, a preferred method in many cases is to stitch the stringer flanges and intercostal flanges to the skin, because stitching provides a greater out-of-plane peel strength between the flanges and the skin than is typically attainable using alternative techniques such as co-curing (i.e., relying on matrix bonding between the flanges and the skin).
Currently, stitching of preforms in this manner is performed using two- or three-degree-of-freedom gantry-type stitching machines. For example, a wing skin preform is laid horizontally on the work surface of the machine and the stringers and intercostals are positioned on the skin in their proper locations, and the stitching device is moved by the machine generally along horizontal directions with the needle moving along a vertical axis so as to stitch the stringer and intercostal flanges to the skin. Where the portion of the preform being stitched is perfectly flat, a two-degree-of-freedom machine is sufficient, since there is no requirement for varying the height of the stitching device relative to the work surface. However, where the preform is contoured so that its height varies along the path over which the stitching device is carried, a three-degree-of-freedom machine is required. These types of machines are complex and expensive.
A further drawback to three-degree-of-freedom machines is that they provide no capability for the needle to be rotated relative to the preform. Accordingly, in portions of the preform where the height of the preform varies along the stitching path, the needle must penetrate the preform in a direction that is not perpendicular to the surface of the preform, which can make stitching more difficult and also cause problems with part quality. More particularly, when the needle passes through the preform in a non-perpendicular direction, the needle must penetrate a greater total thickness of fiber plies than if it penetrated perpendicularly, and thus a longer needle is required and the resistance to the needle is greater. The needle is therefore more prone to problems of excessive bending. Additionally, tensile forces of the thread acting non-perpendicularly tend to pull the fiber plies out of alignment. One possible solution to the problem of needle non-perpendicularity is to use a five-degree-of-freedom gantry-type machine so that the orientation of the needle axis can be changed. Unfortunately, such machines are even more expensive and complex than three-degree-of-freedom machines.
Furthermore, it would be desirable in many cases to be able to stitch parts whose surfaces are essentially vertical, requiring that the needle move along a horizontal axis. For instance, it would be desirable to be able to stitch intercostal flanges to the vertical stringer webs of a composite wing preform. Currently, the intercostal flanges are affixed to the stringer webs by co-curing. Stitching of the intercostal flanges to the stringer webs would provide substantially higher-strength attachment of these parts. However, none of the known commercially available gantry-type machines are capable of stitching vertically oriented parts such as intercostal flanges and stringer webs.